The father-of-four - also known as Siddhartha Dhar - fled the UK to join IS in 2014 despite being on the authorities' radar, having been arrested six times.

"My name's Abu Rumaysah," he says in the first trailer for the documentary. "One day when Sharia comes, you'll see this black flag flying everywhere," he adds as he poses next to a black flag.

Chilling scenes with the flag (Photo: Channel 4)

His two associates, who feature in the documentary, tell the cameras his "legacy lives on" here, claiming that they are "brainwashing" youngsters with their "deadly" message.

In more scenes, Shamsuddin claims he can't work because of chronic fatigue syndrome, as he's seen preaching on the streets.

He is seen shouting into a microphone at one point, saying: "David Cameron will be arrested for the way he has treated Muslims."

However, despite his shocking views, he later admits he is a fan of The Great British Bake Off - but adds that he doesn't think recent winner Nadiya Hussain is a real Muslim.

An inside look on our screens (Photo: Channel 4)

Nadiya Hussain won the nation's hearts on The Great British Bake Off (Photo: BBC)

He goes on to praise Dhar as "a very good Muslim - he's from the best of the best of the best."

The documentary also shows Mr Haleema talk about grooming his beard with oil, and director Jamie Roberts said there is very much a style followed among the men which he described as a "Nike Air Jihadi look".

He goes on to tell the camera of his plans to have homosexuals "chucked off high buildings" to recreate ISIS executions, as well as adulterers stoned to death on a green in Ealing, West London.

He is seen speaking to crowds (Photo: Channel 4)

Haleema later blames the Paris attacks on the French, claiming: "The chickens have come home to roost isn’t it, so it’s something they brought upon themselves.

"This is what happens in war isn’t it, obviously we don’t condone the killing of innocents, but this is what happens in war."

Meanwhile Shamsuddin admits: "Our message is deadly, we are calling for world domination, and for Shariah for the UK, and for that alone, our message is quite deadly."

Shocking signs revealed in the doc (Photo: Channel 4)

Channel 4 are now airing the groundbreaking documentary that looks behind the headlines and explores why Abu, and others like him, have turned to ISIS and become British jihadis.

Director Jamie Roberts said he had "chills" when he heard the voice of Siddhartha Dhar in the recent IS propaganda video.

In the programme, father-of-four Rumaysah is seen in January 2014 holding black flags in his garage and talking to Roberts.

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Rumaysah, who says he is from a Hindu background and converted when he was about 19, said the notion that the black flag of Islam would one day fly high over 10 Downing Street may have been described as "ludicrous" 10 or 15 years ago.

But he said that it is now "a very real, real possibility the way Muslims are coming forward in this country".

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Rumaysah was held in September 2014, when he was 31, as part of an investigation into alleged support of the banned extremist group al-Muhajiroun. He left Britain with his family the day after being released on bail, travelling to Paris and then Syria.

It has not been officially confirmed that the masked militant in the video, released a couple of weeks ago, is Rumaysah. The video shows the murder of five men accused by IS of spying for the UK.

When the recent IS video emerged, Mr Roberts received a text from Mr Shamsuddin, which had a link to the video, and a message saying: "You may know the voice."

The director felt "chills"

Speaking about how he felt at that time, Mr Roberts said it almost felt "unreal".

He said he had spent quite some time with Rumaysah, including a few hours off camera where they talked in a coffee shop.

"I'd got to know him fairly well," he said, adding that the fact there was a possibility the man in the IS video was Rumaysah seemed "crazy".

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Mr Roberts said that having met Rumaysah in Walthamstow and knowing about his job of renting out bouncy castles, he was left with a feeling of "how can this guy ever be a threat?"

He said he still finds it difficult to comprehend "a journey of that magnitude".

Mr Roberts said he thought Rumaysah's talk about the black flag flying above Downing Street was "fantasy". He said: "The black flag over Downing Street ... I don't believe that's ever going to happen but someone might try and do it. And it's that trying to is what's difficult and I think that's what the police are trying to deal with at the moment."

An inside look at the sickening lives

The film maker said the people he spoke to for his documentary seem to think "it's a matter of time" before an attack will happen.

Mr Shamsuddin and Mr Haleema have yet to see the final cut of the documentary, and Mr Roberts thinks they may "grumble", but he said he will be able to "call them out on it".

The documentary, which was filmed over two years, shows Muslims stopping to challenge extreme views. Mr Roberts said people need to understand that the Muslim community is coming out and opposing extremism.